Magneto-inductive flow measuring devices utilize, for volumetric flow measurement, the principle of electrodynamic induction: Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube; it is thus proportional to the volume flow rate. If the density of the medium is known, then the mass flow in the pipeline, or in the measuring tube, can be determined. The measurement voltage is usually sensed via a measuring electrode pair, which is arranged, as regards coordinates, along the measuring tube axis in the region of maximum magnetic field strength and where, thus, the maximum measurement voltage is to be expected. The measuring electrodes are usually galvanically coupled with the medium; however, also magneto-inductive flow measuring devices with capacitively coupling measuring electrodes are known. Subsequently herein, exclusively measuring electrodes coupling galvanically with the medium will be discussed.
The measuring tube can be manufactured either of an electrically conductive material, e.g. stainless steel, or it is composed of an electrically insulating material. If the measuring tube is manufactured of an electrically conductive material, then it must be lined with a liner of an electrically insulating material in the region coming in contact with the medium. The liner is composed usually of a thermoplastic- or a thermosetting-plastic or an elastomeric, synthetic material. However, also magneto-inductive flow measuring devices equipped with a ceramic lining are known.
The measuring electrodes are, besides the magnet system, fundamental components of a magneto-inductive flow sensor. In the construction and arrangement of the measuring electrodes, it is to be assured, that they can be assembled in the measuring tube as simply as possible and that, subsequently in measurement operation, no sealing problems occur; moreover, the measuring electrodes should be distinguished by a sensitive and, simultaneously, low-disturbance registering of the measurement signal.
Fundamentally, two types of electrode constructions can be distinguished:                Measuring electrodes, which are inserted and assembled from the outside into the measuring tube; and        measuring electrodes, which are inserted and assembled from the inside into the measuring tube.        
In the case of the first type, the structure of the electrode head is limited by the diameter of the bore: Assembly from the outside is only possible, when the diameter of the electrode head is smaller than, or equal to, the diameter of the bore in the wall of the measuring tube, into which the measuring electrode is placed. Preferably, measuring electrodes assembled from outside are rod electrodes, such as are described, for example, in EP 0 892 252 A1. Preferably, rod electrodes are used in the case of magneto-inductive, flow measuring devices of small nominal diameter, thus in cases, where assembly from the inside is difficult or not at all feasible, due to the small dimensions. Furthermore, it is, however, also quite acceptable to use rod electrodes in the case of flow measuring devices of greater nominal diameters, since rod electrodes with a ‘Christmas tree’ structure in the region of the electrode shaft can be used, such as disclosed in European patent, EP 0 892 252 A1, without great effort, in a measuring tube of synthetic material or in a measuring tube having a liner of plastic, or synthetic material.
In the case of the second type of measuring electrode, the limitation as regards the dimensioning of the electrode head is absent; here, the electrode shaft must only be so embodied, that it can be assembled in the bore. The electrode head can, here, depending on application, vary both in size as well as also in form, within broad limits. Known, internally mountable, measuring electrodes have, usually, e.g. mushroom-, lens- or plate-shaped, electrode heads. Both types of measuring electrodes are used by the assignee in its flow measuring devices.
British Patent, GB 2 298 716 A discloses two different variants of measuring electrode designs. In the case of the first embodiment, the mushroom-shaped measuring electrode is secured in a bore of the measuring tube lined with an electrically insulating material via a spring washer and a screwed connection. In order to avoid, that medium can penetrate into the bore, there is provided, on the inner surface of the mushroom-shaped electrode head, a circumferential edge, which presses into the liner material in the assembled state. In the case of the second variant, the sealing action is achieved through a sharp-edged, frustoconically-shaped thickening in the region of the electrode shaft. Also in the case of this embodiment, the securement of the measuring electrode in the bore occurs via a Belleville spring and a screwed connection.